This research focuses on the co-development and co-regulation of the sleep and emotion processes during early childhood (ages 2-5 years). The specific aims of this research are (1) to examine the early developmental progression of sleep homeostasis (i.e., accumulation and dissipation); (2) to examine early developmental changes in the circadian process (i.e., phase, amplitude, phase angle of entrainment); and (3) to determine the impact of acute sleep restriction on physiological and behavioral measures of emotion regulation. Given the prevalence of early emotional/behavioral problems (7%-24%) and their stability, identifying factors that may alter pathways to mental illness is critical. Sleep-wake regulation is one such factor. Healthy, normally developing young children (n=40; ages 2-5 years) will participate in four annual 30-day in-home assessments of sleep homeostasis, circadian rhythms, and emotion regulation. Aim 1 addresses developmental changes in the accumulation (build up) and dissipation (decay) of sleep homeostasis. Children are studied while following a strict sleep wake schedule. On five non-consecutive days, the timing of polysomnographic sleep recordings varies as a function of prior wakefulness. EEG spectral power in the slow- wave activity range (.75Hz-4.5Hz) is computed in NREM sleep after 4hrs, 7hrs, 10hrs, 13hrs, and 16hrs of extended wakefulness. Accumulation parameters (time constant, upper asymptote) are estimated by SWA in the first NREM sleep episode of each recording. Dissipation is measured as the time course of SWA during NREM sleep episodes in all-night recordings (13hrs and 16hrs extended wakefulness). Aim 2 addresses developmental changes in circadian measures, including circadian phase [salivary dim-light melatonin onset (DLMO)], urinary melatonin amplitude, and phase angle of entrainment (DLMO-to-bedtime and DLMO-to-wake time intervals). The third aim addresses the effects of sleep restriction on emotion regulation in structured and unstructured home-based contexts. It is hypothesized that sleep restriction will (a) increases negative responsivity; (b) decrease positive responsivity; (c) decrease attention, inhibitory control, and use of adaptive regulation strategies; and (d) heighten physiological responses (respiratory sinus arrhythmia, cortisol reactivity) to emotionally eliciting stimuli/situations